1. Field of the Invention
The present invention relates to an air ridge type semiconductor laser device and a manufacturing method of the semiconductor laser device. In particular, the present invention relates to a long-life air ridge type semiconductor laser device having an excellent characteristic in relation between injection current and light output and a manufacturing method thereof.
2. Description of the Related Art
There are a variety of structures for a waveguide structure of a semiconductor laser device. Among those known structures, an air ridge waveguide type semiconductor laser device, which is often compared with that of an embedded waveguide type semiconductor laser device, has been focused as a waveguide structure which can be easily fabricated. Such an air ridge waveguide type semiconductor laser device (hereinafter, simply referred to as an xe2x80x9cair ridge type semiconductor laser devicexe2x80x9d) can be fabricated in a following manner. First, an upper portion of an upper cladding layer is etched to form a ridge stripe. Due to a thickness of a lower portion of the upper cladding layer, which is left to elongatedly extend outwardly from a lower end edge, a predetermined difference in refractive index in a lateral (horizontal) direction is provided. Thereby, a waveguide having an optical confinement structure in the lateral direction can be realized. The air ridge type semiconductor laser device has advantages in that it is easy to form an optical waveguide, and that it requires less operational current because it is based on real refractive index waveguide which brings small internal loss.
Now, with reference to FIG. 6, an AlGaInP-containing air ridge type semiconductor laser device is taken as an example to explain a structure of a conventional air ridge type semiconductor laser device. FIG. 6 is a cross sectional view showing the structure of the conventional air ridge type semiconductor laser device. The conventional air ridge type semiconductor laser device 50 has a multi-layer structure including an n-GaAs substrate 52, and epitaxially grown layers are formed thereon in the sequential order of a first cladding layer (lower cladding layer) 54 of n-AlGaInP, an active layer 56 of GaInP, a second cladding layer (upper cladding layer) 58 of p-AlGaInP, an intermediate layer 59 of p-GaInP and a contact (capping) layer 60 of p-GaAs.
In the multi-layer structure, the contact layer 60, the intermediate layer 59 of p-GaInP and an upper portion of the second cladding layer 58 are etched to be a ridge stripe 62.
In other words, the second cladding layer 58 comprises an upper layer 58a which, together with the contact layer 60 and the intermediated layer 59, forms the ridge stripe 62, and a thin lower layer 58b which is positioned below the upper layer 58a and elongatedly extends from both bottom ends of the upper layer 58a outwardly. A dielectric film, for example, an SiO2 film is stacked as a protection layer 64 on an upper surface of the lower layer 58b and side planes of the ridge 62. However, an upper surface of the ridge 62 to be a current injection region is not covered with the protection layer 64. In addition, a p-electrode 66 is formed on the protection layer 64 and on the contact layer 60 which is exposed from the protection layer 64, and n-electrode 68 is formed on a back surface (bottom surface in the figure) of the GaAs substrate 52.
Such a conventional air ridge type semiconductor laser device has a fatal problem that the period during which the device can operate exhibiting a predetermined operational property, that is, the device has a short life. For example, there is a problem that when a fixed light output is required, the longer the operating period is, the higher the operational current becomes. In other words, if the operational current is constant, the light output diminishes as the operating period becomes longer. This problem has been noticeably found in air ridge type semiconductor laser devices made of materials containing AlGaAs or AlGaInP, in particular.
As a result, it is difficult to use the air ridge type semiconductor laser device in a field of a light source for an optical pickup used in an optical disk recording/reproducing apparatus, for example, which requires high reliability. Accordingly, under the present circumstances, the air ridge type semiconductor laser devices are used only in a field of a laser pointer, for example, which requires relatively low reliability.
Accordingly, there is a need for a high-reliable long-life air ridge type semiconductor laser device.
During the study of the conventional air ridge type semiconductor device, the Inventors have conceived that one of the factors of the short life of the air ridge type semiconductor laser device is brought by follows. That is, since an interface between the cladding layer exposed by etching and the dielectric film or an ohmic metal (a metal used for an ohmic electrode) at the formation of the ridge is chemically instable, at the time of laser operation, operational current injection accelerates the deterioration of crystals in the cladding layer. In consideration thereof, the Inventors have hit upon an idea to have an epitaxially grown layer having a lattice constant close to that of the second cladding layer on the second cladding layer, as a protection layer, so as to chemically stabilize the second cladding layer. After carrying out various experiments based on the idea, the Inventors have invented the semiconductor laser device and the manufacturing method thereof, which are claimed in the application.
According to a first aspect of the present invention, there is provided an air ridge type semiconductor laser device comprising a structure including an active layer sandwiched between a first cladding layer (lower cladding layer) and a second cladding layer (upper cladding layer) each having a conductivity type different from each other. The second cladding layer comprises an upper layer which forms a ridge stripe, and a lower layer positioned below the upper layer, which elongatedly extends from both lower ends of the upper layer outwardly. A protection layer is provided on an upper surface of the lower layer of the second cladding layer and side planes of the ridge stripe other than an upper surface thereof. In the air ridge type semiconductor laser device, the protection layer is a compound semiconductor layer epitaxially grown on the upper surface of the lower layer of the second cladding layer and the side planes of the ridge stripe.
According to the present invention, the epitaxially grown layer is provided as the protection layer so that crystals of the second cladding layer are prevented from deterioration. Accordingly, the conventional problem that, when a constant light output is required, a longer operating period raises the operational current, that is, the problem that, if the operational current is constant, the longer operating period lowers the light output does not occur in the present invention.
The present invention may be applied without any limitation to materials for the compound semiconductor multi-layer structure which forms the structure in which the active layer is sandwiched between the cladding layers having different conductivity types. In addition, there is no limitation to the composition of the second cladding layer.
It is preferable that the compound semiconductor layer constituting the protection layer is a compound semiconductor layer having a conductivity type different from that of the second cladding layer. According to the arrangement above, the protection layer functions as a current confinement region due to p-n junction separation, which leads to better injection current-light output characteristics.
In addition, since it is required to have the protection layer epitaxially grown on the lower layer of the second cladding layer and side planes of the ridge, in order to have the protection layer and the second cladding layer lattice-matched, the lattice constants of those layers are preferably close to each other. More preferably, the difference between the lattice constant of the protection layer and that of the second cladding layer is 6% or less of the lattice constant of the second cladding layer. For example, in a case where the structure in which the active layer is sandwiched between cladding layers each having a conductive type different from each other is constituted with an AlGaInP-containing compound semiconductor, that is, in a case where the second cladding layer is a layer of AlGaInP, the protection layer preferably comprises GaAs or GaInP.
Film thickness of the epitaxial compound semiconductor layer constituting a protection layer is 0.15 xcexcm or more and 0.3 xcexcm or less.
The Inventors have confirmed through their experiments that the effect of the present invention can be sufficiently achieved if the protection layer has a thickness of 0.15 xcexcm or more. Because it is a function of the protection layer to stabilize a surface condition of the second cladding layer which has an etching surface which is chemically unstable on a surface thereof, a film thickness enough for stabilizing the etching surface is sufficient for the function of the protection layer.
Accordingly, a film thickness which can be uniformly epitaxially grown by using MOCVD (Metal Organic Chemical Vapor Deposition) method or the like is the lower limit for the protection layer. In other words, 0.15 xcexcm or more is enough for the thickness of the protection layer.
On the other hand, 0.3 xcexcm for the upper limit of the protection layer is based on the following thoughts. In consideration of the manufacturing process of the semiconductor laser device, the protection layer is desirable to have a composition which can be epitaxially grown on the second cladding layer by the selective area growth method. For example, if the second cladding layer comprises AlGaInP, the protection layer is preferably comprised of GaAs or GaInP. In a case where these materials are employed in an AlGaInP-containing laser multilayer structure, the difference between a lattice constant of the second cladding layer and that of the protection layer can be 0.6% or less of the lattice constant of the second cladding layer, which is a requirement for a good epitaxial growth.
A band gap of these compound semiconductor layers is smaller than a laser oscillation wavelength of the semiconductor laser device, for example, 650 nm in AlGaInP-containing compound semiconductor. Accordingly, the light loss due to the light absorption by the compound semiconductor protection layer offsets reduction of the internal loss which is a merit of the air ridge waveguide structure. From this point of view, an appropriate thickness of the protection layer is 0.3 xcexcm or less. Conversely speaking, it is difficult to further improve the effect of the present invention with the protection layer having a thickness more than 0.3 xcexcm.
It is preferable that the lower layer of the second cladding layer has a thickness of 0.6 xcexcm or less. The thickness of the lower layer of the second cladding layer, which forms a low refractive index region, is determined in accordance with the following thoughts. It has not been made clear how the operational characteristics of the semiconductor laser device are deteriorated, in a narrow sense. One of the factors of the operational characteristics deterioration is considered to be a model in which a natural emitted light from an active layer is recombined at an interfacial energy level to encourage increase of defects. If such a consideration is correct, the lower layer of the second cladding layer, if only it is as thick as a cladding layer of a usual semiconductor laser device, exhibits the effect of the present invention.
Accordingly, since a general thickness of a cladding layer of a semiconductor laser device is within a range of about 1 xcexcm to 2 xcexcm, the compound semiconductor protection layer is effectively used in a semiconductor laser device in which the lower layer of the second cladding layer has a thickness of 2 xcexcm or less. In particular, in a case where the thickness of the lower layer of the second cladding layer is 0.6 xcexcm or less, an effect of forming a refractive-index waveguide is improved and a low-current operation peculiar to the air ridge structure is enabled so that the effectiveness of the present invention is improved.
According to the present invention, by providing an epitaxially grown compound semiconductor layer as a protection layer on a surface of the lower layer of the second cladding layer and side planes of the ridge, an operational lifetime of the air ridge type semiconductor laser device becomes significantly longer than a conventional air ridge type laser device. In the semiconductor laser device according to the present invention, since the injection current-light output characteristics are improved, the operational current with regard to the same light output is lower in comparison with the conventional air ridge type semiconductor device.